The present invention relates to a circuit substrate and a method for fabricating the circuit substrate which are adaptable to higher density and higher speed.
Recently, semiconductor parts used in computers, etc. are rapidly made increasingly adaptable to higher density and higher speed. Circuit substrates as well are accordingly required to be adaptable to the higher density and the higher speed.
One example of the conventional circuit substrates will be explained with reference to FIGS. 9A and 9B. FIG. 9A is a sectional view of the mounted conventional circuit substrate, which shows the structure of the circuit substrate. FIG. 9B is a perspective view of the circuit substrate, which shows the structure of the circuit substrate.
As shown in FIG. 9A, the circuit substrate 100 is mounted on a printed circuit board 106. The circuit substrate 100 and the printed circuit board 106 are electrically connected to each other via solder balls 104a, etc. Decoupling capacitors 108 are formed on the upper surface of the circuit substrate 100. An LSI substrate 110 is mounted on the circuit substrate 100 mounted on the printed circuit board 106. The circuit substrate 100 and the LSI substrate 110 are electrically connected to each other via solder balls 104b, etc.
As shown in FIG. 9B, through-holes 114 are formed in the circuit substrate 100 at a certain pitch. Via electrodes 116 of a metal are buried in the through-holes 114. Usually, passive elements, active elements, etc., such as decoupling capacitors, etc., are formed on the circuit substrate 100 mounted on the printed circuit board 106. In FIG. 9B, these elements, etc. are omitted.
Prescribed wiring of the LSI substrate 110 and prescribed wiring of the printed circuit board 106 are electrically connected via the via electrodes 116, electrode pads 102a, 102b and solder balls 104a, 104b. 
In the above-described conventional circuit substrate, when the adhesion between the via electrodes of the buried metal and the inside walls of the through-holes is insufficient, the via electrodes often come off. In order to increase the adhesion between the via electrodes and the inside walls of the through-holes, the surface roughness of the inside walls of the through-holes are increased, whereby the detachment of the via electrodes are prevented.
However, when a base material of the circuit substrate is a conducting substrate, such as a silicon substrate or others, the rough inside walls of the through-holes tends to cause leak current. That is, when a conducting substrate is used, an insulation film is formed on the inside walls of the through-holes to ensure the insulation between the via electrodes and the substrate. In this case, when the surface roughness of the inside walls of the through-holes is increased, electric fields are concentrated on the insulation film and the dielectric breakdown is occurred. Resultantly, leak current more tends to be caused.
As a result of increasing the surface roughness of the inside walls of the through-holes, processing strains are present in the inside walls of the through-holes, etc. Resultantly, cracks tend to take place in the circuit substrate.
Furthermore, a metal, etc. whose thermal expansion coefficients are different from that of the substrate is buried in the through-holes, which causes stresses in the substrate. These stresses are a cause for damaging the circuit substrate. However, it is difficult for the conventional circuit substrate to mitigate the stresses caused therein.